Methane is produced by the thermal and biogenic processes responsible for converting organic matter to various solid carbonaceous subterranean materials such as coals and shales. The mutual attraction between the carbonaceous solid and the methane molecules frequently causes a large amount of methane to remain trapped in the solids along with water and lesser amounts of other gases which can include nitrogen, carbon dioxide, various light hydrocarbons, argon and oxygen. When the trapping solid is coal, the methane-containing gaseous mixture that can be obtained from the coal typically contains at least about 95 volume percent methane and is known as "coalbed methane." The world-wide reserves of coalbed methane are huge.
Coalbed methane has become a significant source of the methane distributed in natural gas. Typically, coalbed methane is recovered by drilling a wellbore into a subterranean coalbed having one or more methane-containing coal seams that form a coalbed. The pressure difference between the ambient coalbed pressure (the "reservoir pressure") and the wellbore provides a driving force for flowing coalbed methane into the wellbore. As the ambient coalbed pressure decreases, methane is desorbed from the coal. Unfortunately, this pressure reduction also reduces the driving force necessary to flow methane into the wellbore. Consequently, pressure depletion of coalbeds becomes less effective with time, and is generally believed capable of recovering only about 35 to 50% of the methane contained therein.
An improved method for producing coalbed methane is disclosed in U.S. Pat. No. 5,014,785 to Puri, et al. In this process, a methane-desorbing gas such as an inert gas is injected through an injection well into a solid carbonaceous subterranean formation such as a coalbed. At the same time, a methane-containing gas is recovered from a production well. The desorbing gas, preferably nitrogen, mitigates bed pressure depletion and is believed to desorb methane from the coalbed by decreasing the methane partial pressure within the bed. Recent tests confirm that this process yields increased coalbed methane production rates and suggest that the total amount of recoverable methane may be as high as 80% or more.
Puri et al. also disclose in the above-mentioned U.S. Pat. No. 5,014,785 that air can be injected into a solid carbonaceous subterranean formation to increase methane production. However, injecting an oxygen-containing gas such as air into a coalbed can present several operational problems. For example, the presence of oxygen can cause or increase corrosion-related problems in process equipment such as pumps, compressors and well casings. Also, feeding oxygen-containing fluids into an injection well may form explosive or flammable gas mixtures in the injection well that would not be formed if a gas such as nitrogen was injected into the well. These potential problems may be minimized by reducing the oxygen content of air before injecting air into a formation such as coalbed. One such example of operation with a reduced oxygen content stream is disclosed in Puri, et al., U.S. Pat. No. 5,133,406. The '406 patent discloses depleting the oxygen content of air before injecting the air into a coal seam by inputting air and a source of fuel, such as produced methane, into a fuel cell power system, generating electricity, and forming a fuel cell exhaust comprising oxygen-depleted air.
Co-filed U.S. Ser. No. 08/147,111, which is hereby incorporated by reference, discloses increasing production of methane from solid carbonaceous subterranean formations, such as coalbeds, by processing a gas containing oxygen in a membrane separator, withdrawing oxygen-depleted effluent from the separator, and injecting oxygen-depleted effluent into the solid carbonaceous subterranean formation.
Co-filed U.S. Ser. No. 08/147,125, which is hereby incorporated by reference, discloses increasing the production of methane from solid carbonaceous subterranean formations, such as coal seams, by using a pressure swing process to produce an oxygen-depleted gas.
While the foregoing processes provide improved methods for recovering a methane-containing process stream from solid carbonaceous subterranean formations, the production of the required oxygen-depleted stream is expensive and may in some cases render the economics of the process unfavorable.
In some cases, the foregoing processes may also be economically unfavorable because gaseous components of the injected gas such as nitrogen must be separated from the recovered methane before the methane can be transported through a natural gas pipeline or otherwise utilized.
What is needed is an improved process for the recovery of methane from solid carbonaceous subterranean formations that minimizes the economic impact of the production of oxygen-depleted injectants. Preferably, the process should also mitigate the need to remove injected oxygen-depleted gas from the methane-containing mixture removed from the formation.